Urea synthesis processes, which comprise reacting ammonia and carbon dioxide in an urea synthesis column at a urea synthesis pressure and a urea synthesis temperature, separating ammonium carbamate from the thus-obtained urea synthesis solution at a pressure substantially equal to the urea synthesis pressure as a gaseous mixture of ammonia and carbon dioxide, causing an absorption medium to absorb said gaseous mixture so as to recycle the absorbed medium to the urea synthesis column, and separately providing an urea solution from the urea synthesis solution from which said ammonium carbamate has been separated, are well known, and a variety of proposals have been made with regard to the processes. In the processes, since liquid is allowed to flow down from the condenser to the urea synthesis column and from the urea synthesis column to the stripper by head difference, it is necessary to place the condenser and urea synthesis column at high grounds.
It is well known that the above-described processes employ corrosion-preventive oxygen supplied to the urea synthesis column, condenser and stripper as a countermeasure to corrosion. The oxygen, being dissolved in the urea synthesis solution, is supplied onto anticorrosive films in the interior of the equipments, and therefore the interior of the equipments was protected from corrosion.
As described above, even if the interior was immersed in a highly corrosive recovery solution or urea synthesis solution, corrosion prevention was possible as far as corrosion-preventive oxygen was supplied, although it depended on the operating pressure and temperature, liquid conditions and other conditions.
By reasons of much time and labor taken to install and repair the equipments, there have been developed urea synthesis processes wherein urea synthesis column and other equipments are placed on the ground.
The applicant previously proposed, in Japanese Patent Application No. 234364/1997, a urea synthesis process which permits placing the synthesis column and other equipments on the ground.
The proposal relates to a urea synthesis process and an apparatus therefor in which a urea synthesis column is installed in a lower part and a vertical condenser is installed thereon or thereabove, a first downpipe is provided to make the condenser communicate with the urea synthesis zone, the condensate is allowed to flow down by gravity from the top part of the condenser through the down pipe to the bottom part of the urea synthesis zone, and the condensate thus having flowed down is allowed to flow down by gravity through a second down pipe having an opening in the top part of the urea synthesis column to the top part of a stripper, or to a process in which the condensate from a vertical condenser is sucked by an ejector using heated feed liquid ammonia as a driving fluid and introduced into the bottom part of the urea synthesis column where it is subjected to urea synthesis, and an apparatus therefor.
In the above processes, a recovery solution is supplied to the shellside of the condenser. The operating conditions in the condenser in the former process are such that the pressure is the same as that of the urea synthesis, the molar ratio of ammonia to carbon dioxide (referred to as N/C hereunder) is controlled at 2.5-4.5, the molar ratio of water to carbon dioxide (referred to as H/C hereunder) at 0.0-1.0, the residence time at 10-30 minutes, and the operating temperature is controlled to be 170-190.degree. C., so that a urea synthesis rate of 20-60% is achieved.
In the latter process, the pressure is selected from 140-250 bar, the N/C is set at 2.5-3.5, the H/C at 0.0-1.0, the residence time at 10-30 minutes, and the operating temperature is controlled to be 130-250.degree. C., preferably 170-190.degree. C., so that a urea synthesis rate of 20-60% is achieved.
Another example suggesting to place equipments on the ground includes a urea production process disclosed in Japanese Patent Laid-Open Publication No. 209555/1985. The publication discloses that condensation in the condensing zone is carried out in an immersed condenser, particularly, in the shellside of a tubular heat exchanger arranged horizontally and also that the condensation can be effected in a condensing zone integrated with a reaction zone. According to the publication, in the condensing zone, the pressure is selected from 125-350 bar which are equivalent to the synthesis pressures, and at least 30% of the achievable equilibrium amount of urea is formed. These descriptions and examples teach that the operating temperature is 130 -150.degree. C.
In the foregoing prior art, it is possible to synthesize urea in the shellside of the condenser. In any case, regardless of whether the condenser is installed vertically or horizontally, a tube plate and cooling tubes in the condensing zone are immersed in a highly corrosive solution comprising ammonia, ammonium carbamate, urea and water in the foregoing prior art. The condenser is in the conditions of pressure and temperature capable of synthesizing urea and therefore is under an environment where below-described crevice corrosion and corrosion of joints including weld joints of the tube plate and the cooling tubes were apt to take place.
As it is well known, the condenser has hundreds to a thousand or more of cooling tubes installed therein, although the number varies with production. Therefore, when below-described troubles occurred, it took much time and effort to discover the trouble spots. Further, when one tried to repair the trouble spots during peridic maintenance, it was practically impossible to repair them from the shellside, because there was no space available for men to enter the equipment and repair the spots.
As described above, the condenser has cooling tubes installed in a tube plate, and as one of the installation methods, there is mentioned a tube expansion method or a method in which tube expansion and sealweld with tube plate are combined. In these methods, it is well known that minute crevices evolve between the tube plate and the cooling tubes upon fabrication. When a recovery solution or urea synthesis solution creeps into the crevices, the solution remains in the crevices. Under such conditions, corrosion-preventive oxygen supplied to the aforesaid anticorrosive films is deficient. As a result, an environment is formed where corrosion called crevice corrosion is liable to occur between the tube plate and the tubes, and troubles are caused such that part of the cooling tubes are broken or the tube plate is damaged by corrosion.
When part of the cooling tubes were broken, there was no particularly effective countermeasure other than filling plugs in the broken tubes to stop the cooling functions of the tubes. Especially, when the tube plate was damaged by corrosion, no repairing method could be applied.
In view of the above circumstances, as one of the methods for avoiding crevice corrosion, a method has been practiced wherein a tube plate is directly welded to cooling tubes by internal bore welding or internal bore butt welding (hereinafter referred to as internal bore welding). However, under the operating conditions of a pressure of 150-300 bar and a temperature of 150-200.degree. C. in the condenser, the weld joints are immersed at all times in a highly corrosive recovery solution and joined solution thereof with the condensate and subjected also to heat history. Accordingly, since these factors are entwined with each other complicately, it has been impossible to prevent completely troubles, such as crack of the weld joints due to heat history, corrosion due to the highly corrosive recovery solution, and compounded crack and corrosion due to heat history and the recovery solution.
When such troubles occurred, there was no practically effective countermeasure except for stopping operation for a while and filling plugs in the damaged cooling tubes to suspend the function of the tubes.
Even when plugs are struck in, wetting of the weld joints is not avoided and the synthesis solution stays in the plug-struck cooling tubes without being replaced, so that corrosion of the tubes increases acceleratingly due to deficiency of dissolved oxygen and even the plugs themselves may also be corroded eventually.
Even if the corresponding spots were rewelded during periodic maintenance, its preparation took much time and costed a great deal, so that this way of maintenance was not so advantageous.
As has been described above, it has been expectd to propose means for preventing troubles such as crack and corrosion of the joints of a tube plate and cooling tubes and of the internal bore weld joints of a tube plate and cooling tubes regardless of in the synthesis column or in the condenser, and a process for synthesizing urea using said means.